title: The stress protein/chaperone Grp94 counteracts muscle disuse atrophy by stabilizing subsarcolemmal neuronal nitric oxide synthase.
abstract: OBJECTIVE
  Redox and growth-factor imbalance fosters muscle disuse atrophy. Since the endoplasmic-reticulum chaperone Grp94 is required for folding insulin-like growth factors (IGFs) and for antioxidant cytoprotection, we investigated its involvement in muscle mass loss due to inactivity.
  
  
  RESULTS
  Rat soleus muscles were transfected in vivo and analyzed after 7 days of hindlimb unloading, an experimental model of muscle disuse atrophy, or standard caging. Increased muscle protein carbonylation and decreased Grp94 protein levels (p<0.05) characterized atrophic unloaded solei. Recombinant Grp94 expression significantly reduced atrophy of transfected myofibers, compared with untransfected and empty-vector transfected ones (p<0.01), and decreased the percentage of carbonylated myofibers (p=0.001). Conversely, expression of two different N-terminal deleted Grp94 species did not attenuate myofiber atrophy. No change in myofiber trophism was detected in transfected ambulatory solei. The absence of effects on atrophic untransfected myofibers excluded a major role for IGFs folded by recombinant Grp94. Immunoprecipitation and confocal microscopy assays to investigate chaperone interaction with muscle atrophy regulators identified 160 kDa neuronal nitric oxide synthase (nNOS) as a new Grp94 partner. Unloading was demonstrated to untether nNOS from myofiber subsarcolemma; here, we show that such nNOS localization, revealed by means of NADPH-diaphorase histochemistry, appeared preserved in unloaded myofibers expressing recombinant Grp94, compared to those transfected with the empty vector or deleted Grp94 cDNA (p<0.02).
  
  
  METHODS
  Grp94 interacts with nNOS and prevents its untethering from sarcolemma in unloaded myofibers.
  
  
  CONCLUSIONS
  Maintenance of Grp94 expression is sufficient to counter unloading atrophy and oxidative stress by mechanistically stabilizing nNOS-multiprotein complex at the myofiber sarcolemma.
authors.affiliation: 1 Institute for Neuroscience , Padova Section, CNR, Padova, Italy .
authors.forename: Maurizio
authors.initials: M
authors.lastname: Vitadello
authors.forename: Jennifer
authors.initials: J
authors.lastname: Gherardini
authors.forename: Luisa
authors.initials: L
authors.lastname: Gorza
mesh_terms: D000818:Animals; D004795:Enzyme Stability; D005260:Female; D008562:Membrane Glycoproteins / Q000378:metabolism*; D020966:Muscular Disorders, Atrophic / Q000201:enzymology / Q000378:metabolism* / Q000473:pathology*; D052248:Nitric Oxide Synthase Type I / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012508:Sarcolemma / Q000201:enzymology* / Q000378:metabolism
chemical_list: D008562:Membrane Glycoproteins; C053387:endoplasmin; D052248:Nitric Oxide Synthase Type I